1. Field of the Invention
The present invention relates to a wavelength-division multiplex communication system and wavelength-division multiplex communication apparatus for use in the system.
2. Description of the Related Art
FIG. 27 shows an example of an existing wavelength-division multiplex (WDM) communication system. Referring to FIG. 27, a WDM communication system 100 has terminal equipment, 200 and 400, coupled to a communication network, such as an ATM (Asynchronous Transfer Mode) network, a SONET (Synchronous Optical Network), an SDH (Synchronous Digital Hierarchy) network, or an Ethernet (a registered trademark of Xerox Corporation), and an optical repeater 300 which is disposed between the terminal equipment, 200 and 400, and is coupled to them through an optical transmission path (an optical fiber such as a SMF (Single Mode Fiber)), 500 and 600. The optical repeater 300 amplifies and relays WDM signals bi-directionally transmitted over the optical transmission path 500 and 600. The number of the optical repeaters (ILA) 300 required depends on a distance over which the WDM signals have to travel.
The terminal equipment (hereinafter simply called “terminal”) 200 (400) has, for example, a transponder (transceiver) 201 (401), a wavelength-division multiplexer (WDM coupler) 202 (402), and an optical amplifier, 203 (403) and 204 (404), such as an EDFA (Erbium Doped Fiber Amplifier). The optical repeater (hereinafter also called “repeater station”) 300 has a relay light amplifier, 301 and 302, such as an EDFA.
In the WDM communication system 100, the transponder 201 (401) of the terminal 200 (400) receives transmission signals through a required network (an ATM network, a SONET, an SDH network, or an Ethernet) and converts the received signals into light signals at predetermined wavelengths (channels). The WDM coupler 202 (402) multiplexes the resulting channels into WDM signals. The optical amplifier 203 (403) then simultaneously amplifies the WDM signals up to predetermined signal levels (power), and the amplified WDM signals are sent out on the optical transmission path 500 (600).
The WDM signals thus sent out on the optical transmission path 500 (600) are input to a repeater station 300. The optical amplifier 301 (302) simultaneously amplifies the weakened WDM signals up to desired signal levels, thereby compensating for losses caused during transmission, and the amplified WDM signals are received by an opposing terminal equipment 400 (200) on a receiving end. In the receiver terminal 400 (200), an optical amplifier 404 (204) simultaneously amplifies the WDM signals, which have been received through the optical transmission path 500 (600), once again, thereby compensating for losses caused during transmission. A demultiplexer 405 (205) separates the amplified WDM signals into channels at wavelengths, and each of the thus separated channels is converted into an electric signal by the transponder 401 (201). The resulting electric signals are then sent out as transmission signals on a desired network, such as an ATM network, a SONET, an SDH network, or an Ethernet.
In such a WDM communication system 100, it is better known that transmission loss varies with the wavelength transmitted, across the transmission band of the optical transmission path 500(600). As a countermeasure, the spectrum of output or input WDM signals is monitored to separately adjust (pre-emphasis control) the transmission levels of light signals at different wavelengths (channels), so that a tilt caused in the WDM signals can be compensated for.
For this purpose, as shown in FIG. 28, for example, in the terminal 200 (400), the optical coupler 208 (408) and 209 (409) splits off part of the output of optical amplifiers 203 (403) and 204 (404), respectively, into a spectrum analyzer (SAU: Spectrum Analyzer Unit) 210 (410), where the monitoring of the spectrum of the output or input WDM signals is performed. Likewise, in the repeater station 300, as shown in FIG. 29, the optical couplers 304 and 305 split off part of outputs of the optical amplifiers 301 and 302, respectively, into an SAU 306, where the monitoring of the spectrum of the output or input WDM signals is performed.
In accordance with the monitoring result, a CPU 706 of the SAU, 210 (410) and 306, (described later) generates required monitoring control information (pre-emphasis setting information, or the like), and this monitoring control information is superimposed on an optical channel that is previously assigned as an OSC, by an OSC section 219 (419) in the terminal 200 (400) and by an OSC section 307 (or 310) in the repeater station 300. An optical coupler, 220 (420) and 308, then inserts the optical channel as part (an OSC channel signal) of the WDM signals to be transmitted on an optical transmission path, 500 and 600.
In the terminal 200 (400) of FIG. 28, the reference number “206 (406)” designates an optical variable attenuator for controlling the transmission level of a transmission light signal input from the transponder 201 (401). The reference number “207 (407)” designates a photodiode (PD) for receiving the light signal, which has been controlled in transmission level by the optical variable attenuator 206, and converting the light signal into an electric signal, and then inputting the resulting electric signal both into an optical circuit 212 (412) (described later) and into an OSC 219 (419). The reference number “221 (421)” designates an optical coupler for splitting off part of the WDM signals, which is received through the optical transmission path 600, into the OSC section 219 (419).
Further, in the repeater station 300 of FIG. 29, the reference number “303 (309)” designates an optical coupler for receiving the WDM signals input through the optical transmission path 500 (600) and splitting off part of the WDM signal into an OSC section (309) for the purpose of receiving an OSC channel signal. The reference number “311” designates an optical coupler for inserting an OSC channel signal into an output WDM signal to be sent out on an optical transmission path 600.
Concretely, each of the SAU 210 (410) of the terminal 200 (400) in FIG. 28 and the SAU 306 of the repeater station 300 in FIG. 29, has a switch 701, an optical circuit employing a PD array 703, an analog amplifier 704, an AD (Analog to Digital) converter 705, a CPU 706, a bias circuit 707, and a DA (Digital to Analog) converter 708.
With this construction, in the SAU, 210 (410) and 306, in response to an instruction given by the CPU 706, the switch 701 selects one of the two inputs, the WDM signals to be sent out on an optical transmission path 500 and the WDM signals to be sent out on an optical transmission path 600, to output to an optical circuit 702. The input WDM signals at separate wavelengths are then converted by the PD array 703 into electric signals. At that time, the CPU 706 adjusts, if necessary, a bias applied to the PD array 703, via the DA converter 708.
The analog amplifier 704 amplifies the thus obtained electric signals up to desired levels, and the AD converter 705 then converts the electric signals into digital form to input to the CPU 706. The CPU 706 analyzes the spectrum of the input WDM signal on each channel, or each WDM signal to be sent out on an optical transmission path 500 or 600, to generate required monitoring control information.
The above-described SAU 210 (410), however, employs an highly expensive optical circuit 702 (PD array 703), thereby significantly increasing costs for manufacturing the terminal 200 (400) and the repeater station 300.
Further, since the WDM communication apparatus (terminal 200 (400) or repeater station 300) combines optical channels at separate wavelengths into WDM signals, increase in line traffic on a specific channel causes access congestion in the channel, thus impairing the transmission efficiency. Hence, even in a communication apparatus, such as a WDM communication apparatus, with a large amount of transmission capacity, if the traffic is increased, it will affect better use of the transmission capacity.
Therefore, in a case where an apparatus is coupled to the WDM communication apparatus through a WDM line, the access should be balanced among the channels. As is evident from FIGS. 28 and 29, however, since the WDM communication apparatus itself never converts light signals into electric form, it is impossible for the apparatus to recognize how much traffic the individual channels bear.